The present invention relates to an optical transmission characteristic measuring apparatus for implementing wide dynamic range measurement of the wavelength gain characteristics of optical devices such as semiconductor optical amplifiers and optical fiber amplifiers, the wavelength loss characteristics of fiber gratings, waveguide type gratings, and other types of filters, and the like, and a calibration method using the measuring apparatus.
Conventionally, the optical transmission characteristics of an optical device, e.g., the gain characteristics of an optical fiber amplifier or the loss characteristics of a bandpass filter for extracting a specific wavelength in wavelength division multiplexing (WDM) communication have been measured by one of the following methods:
(1) inputting output light from a wideband light source such as a white light source or SLD (Super Luminescent Diode) to the optical device, and inputting output light from the optical device to an optical spectrum analyzer, thereby measuring the characteristics; and PA1 (2) changing the wavelength of a tunable-wavelength light source in certain steps within the bandwidth, inputting output light from the light source to the optical device, and measuring the output light level of the optical device for each wavelength with an optical power meter. PA1 a tunable-wavelength light source unit which can set a wavelength of output light in accordance with a first wavelength setting signal and inputs the output light to the object; PA1 a spectroscope unit which can set a selected wavelength in accordance with a second wavelength setting signal and performs spectroscopy of light passing through the object; and PA1 a processing unit for setting the wavelength of the output light from the tunable-wavelength light source section and the selected wavelength of the spectroscope unit to an equal wavelength by performing tuning control on the first wavelength setting signal and the second wavelength setting signal, thereby obtaining optical transmission characteristics of the object from an output signal from the spectroscope unit. PA1 setting a wavelength of output light from the tunable-wavelength light source unit; PA1 obtaining a wavelength at a peak level of the wavelength having undergone spectroscopy in the spectroscope unit by wavelength-sweeping the spectroscope unit in a wavelength range including the set wavelength; PA1 causing the processing unit to obtain a wavelength difference between the set wavelength of the tunable-wavelength light source unit and the wavelength at the peak level; PA1 causing the processing unit to obtain wavelength difference data of one of the tunable-wavelength light source unit and the spectroscope unit which corresponds to the wavelength difference; PA1 causing the processing unit to calibrate current wavelength setting data with the wavelength difference data; and PA1 wavelength-tuning the tunable-wavelength light source unit and the spectroscope unit by performing a wavelength tuning process using the processing unit.
The following problems are, however, posed in implementing wide dynamic range measurement of wavelength loss/gain characteristics by the above methods.
Method (1) demands a wideband light source having a very high power or an optical spectrum analyzer having a very high sensitivity and wide dynamic range.
Method (2) demands a tunable-wavelength light source with a very high side mode suppression ratio (SMSR). With the state-of-the-art method, the limit of noise at the peak is 50 dB.